Conventionally, the electrophotographic image forming apparatus using liquid developer has been known. In such an image forming apparatus, an electrostatic latent image is formed on the photoreceptor, and this is developed by liquid developer to a toner image. This toner image is transferred onto paper and is fixed thereon.
The liquid developer is prepared by dispersing toner particles made of resin and pigment into a carrier solution as an insulation solution exemplified by silicone oil in a high concentration. In order to develop an electrostatic latent image with this liquid developer, a thin layer of the developer of microns is formed on a developer carrier such as a development roller, and this thin-film developer is brought into contact with a photoreceptor.
As described above, when the liquid developer is used for development, formation of a uniform thin film of the developer of a predetermined concentration is crucial to obtain images with less variation of image density. To put it another way, it is important to maintain constant concentration of the liquid developer.
One of the techniques known in the conventional art to measure the concentration of the developer to maintain constant concentration of the liquid developer is the technique of calculating the developer concentration by detecting the light transmittance of the developer (Unexamined Japanese Patent Application Publication No. H09-281808 and Unexamined Japanese Patent Application Publication No. H11-73029). However, the method of calculating the developer concentration by light transmittance is accompanied by the problem of poor accuracy in measuring the concentration in the case of highly concentrated developer. This is because, when the developer concentration is low, light transmittance is greatly changed by a change in concentration. However, when the concentration is increased, the light transmittance is reduced to the extent of reaching the point of saturation.
To solve this problem, the U.S. Pat. No. 6,131,001 discloses a method of using the viscosity of the developer to measure the concentration of a highly concentrated developer. The technique disclosed in this document uses, as the method of measuring the viscosity of developer, the method of obtaining the viscosity from the pressure difference of the developer in a pipe, the method of installing a viscometer in the tank accommodating the developer, and the method of obtaining the viscosity from the torque by the flowing developer. However, any of these methods has the problem of complicated structure and lack of sufficient precision.
Further, the Unexamined Japanese Patent Application Publication No. H06-277477 discloses the development of a technique of obtaining the viscosity of highly concentrated slurry (fuel mixture of coal and water) from the rotating torque in stirring operation by rotation, although this is not a liquid developer. However, while the measurement of the rotating torque is comparatively simple, this technique has a defect of requiring the complicated procedure of calculating by detecting the amount of the liquid in a container.